1. Field of the Invention
The present invention relates to a silica glass crucible used for the manufacture of silicon single crystals according to the Czochralski method.
2. Description of the Prior Art
The method of manufacturing silicon single crystals according to the Czochralski method has heretofore been in use and it has become substantially a perfect technique.
As is well known, this technique is so designed that after molten silicon starting material has been contained in a silica glass crucible, a silicon seed crystal is brought into contact with the molten silicon surface and simultaneously the silicon seed crystal is slowly pulled while rotating it, thus growing a silicon single crystal along with the solidification at the contact surface between the pulled silicon single crystal and the molten silicon surface and thereby producing the cylindrical silicon single crystal.
At this time, in order that the silicon single crystal may become a P-type or N-type semiconductor in accordance with the intended use, the silicon starting material is mixed with a suitable amount of such dopant as boron, antimony or phosphorus. The ratio of such dopant taken into the crystal from the molten silicon (i.e., the segregation coefficient) is generally not greater than 1. The concentration of the dopant in the silicon single crystal determines its resistivity and therefore it should preferably be uniform in the crystal.
Also, besides the dopant which is intentionally introduced into the silicon single crystal as mentioned above, the presence of oxygen introduced unavoidably during the manufacture is not negligible. In other words, the concentration of the oxygen which is introduced into the silicon single crystal has a considerable influence on the characteristics and yield of a semiconductor product and the oxygen concentration should also be uniform throughout the upper part to the lower part of the single crystal.
However, as the pulling of the silicon single crystal proceeds, the molten silicon with the crucible is decreased and the concentration of the impurity is varied. In other words, since the segregation coefficient of the dopant is not greater than the dopant concentration of the molten silicon is increased gradually so that the silicon single crystal is varied in dopant concentration from the upper part to the lower part of the crystal. Also, since the oxygen concentration of the molten silicon is dependent on the amount of the oxygen released from the silica glass crucible into the molten silicon, the concentration of the oxygen introduced into the crystal is also varied with decrease in the molten silicon.
As mentioned previously, the quality of the pulled silicon single crystal is varied along the pulling direction. However, the product actually used as wafers is limited to the portions having dopant concentrations and oxygen concentrations in limited ranges. As a result, the extent of the pulled silicon crystal which can be used as the product is extremely limited.
Some methods have been proposed for the solution of these problems and the typical method which can be considered as practical is one employing a double-structure crucible.
In other words, Japanese Patent Publication No. 40-10184 discloses a method in which a concentric crucible adapted to be heated from the outer periphery thereof is so constructed that the molten silicon in an outer crucible is separated from the molten silicon on the inner side by a partition but the two are mutually communicated and a semiconductor crystal is pulled centrally while feeding the semiconductor starting material to the molten silicon on the outer side.
Referring to FIG. 10, there is schematically shown a silicon single crystal manufacturing apparatus employing a double structure crucible, and a crucible 22 and a partition member 23 are constructed integrally by using high-purity silica glass. Numeral 25 designates the molten silicon contained in the crucible 22, and 26 a silicon single crystal pulled from the surface of the molten silicon within the partition member 23. It is to be noted that the lower part of the partition member 23 is formed with a hole 24 to permit the molten silicon 25 to flow between the outer and inner sides of the partition member.
FIG. 10 is a schematic diagram showing the case in which the double structure crucible is incorporated in a batch-type silicon single crystal manufacturing apparatus. The molten silicon of a given dopant concentration is contained on the inner side of the partition member 23 and the molten silicon containing no dopant is contained on the outer side of the partition member 23. It is constructed so that a silicon single crystal 26 is pulled from the single flows to the single crystal growing section from the outer side of the partition member, hereby always maintaining uniform the concentration of the dopant within the single crystal growing section.
Referring now to FIG. 11, there is illustrated another type of construction in which while pulling a silicon single crystal from the single crystal growing section, powder starting material 29 is continuously fed to the starting material feed section from a starting material feed pipe 28 thereby maintaining constant the amount of the molten silicon within the single crystal growing section, and it has the object of maintaining constant the dopant and oxygen concentrations of the molten silicon in the single crystal growing section.
Where the double structure crucible is used to pull a silicon single crystal in accordance with such conventional technique, the heat environment in the molten silicon becomes directly opposite to the case in which the ordinary single structure crucible is used.
In the case of the CZ method using the ordinary single structure crucible, the crucible side wall portion is higher in temperature than the crucible bottom portion. In other words, the amount of heat supplied from the crucible side wall portion is greater than the amount of heat supplied from the crucible bottom portion. Reflecting this fact, it is said that the convection of the molten silicon within the silica glass crucible is predominated by the flows as shown in FIG. 8. If such convection of the molten silicon, there is less temperature variation at the solid-liquid interface between the silicon single crystal and growth is attained.
Where the pulling of a silicon single crystal is effected by using the double structure crucible, however, the amount of heat supplied to the single crystal growing section through the crucible side is supplied indirectly through the starting material feed section and therefore the proportion of the heat input through the crucible bottom is increased as compared with the case where the single-structure crucible is used. As a result, the temperature distribution in the double structure crucible becomes opposite to the case where the single structure crucible is used and therefore the maximum value of the temperature in the silica glass crucible surrounding the single crystal growing section is positioned at the crucible bottom portion. Its temperature distribution is such that the crucible bottom portion is high in temperature and the partition member wall surface is relatively low in temperature. Under such heat environment where proportion of the heat input through the bottom portion is large, the heat convection of the molten silicon within the single crystal growing section may possibly be predominated by the flow field as shown in FIG. 9 which is directly opposite to that shown in FIG. 8. Since such flow field is unstable, the high-temperature molten silicon at the crucible bottom portion is intermittently moved directly to the solid-liquid interface of the silicon single crystal so that the resulting heat variation causes defects in the silicon single crystal to be pulled and hence the occurrence of dislocations.
On the other hand, the pores included in the silica glass crucible may also be considered as a cause for impeding the stable pulling of the silicon single crystal. In other words, while the surface of the silica glass crucible is subjected to erosion by its reaction with the molten silicon, at this time the pores confined within the silica glass crucible break and enter into the molten silicon thus giving rise to a problem that dislocations are caused when the resulting bubbles and the broken pieces of silica glass the solid-liquid interface of the silicon single crystal.